Biological treatment processes are finding application throughout industry. Such processes have been used in waste water treatment, hazardous waste remediation, desulfurization of coal, and biooxidation of refractory sulfide ores.
A variety of methods can be employed in the biological treatment of solid materials, including in situ treatment, landfarming, composting, heap treatment, and stirred or agitated tanks. In the ex situ biological treatment of solid materials, some sort of bioreactor is used to carry out the biotreatment. A bioreactor can be defined as a vessel or body in which biological reactions are carried out by microorganisms, or enzymes they produce, contained within the reactor itself. The main objective in the design of a bioreactor is to generate an optimal environment for the desired biological process to take place on a large and economic scale.
When a solid material is being biotreated, the desired biological reactions typically involve the degradation, either directly or indirectly, of some undesired compound present in the solid material. To accomplish this economically, the bioreactor needs to reduce the concentration of the undesired compound to an acceptable level in an acceptable quantity (in terms of flow rate) of solid material to be treated.
In general biotreatment processes are slow, and if they are aerobic, they require large amounts of oxygen for the aerobic microorganism(s) to metabolize, either directly or indirectly, the undesired compound. Oxygen transfer, therefore, is typically a major problem for the large class of aerobic biological treatment processes available. Current aerobic bioreactor designs attempt to ensure not only that the microorganisms being used have access to the material to be biooxidized or metabolized, but also that all areas of the bioreactor have an adequate oxygen and nutrient supply, as well as maintain the correct pH and temperature, for the biological process to proceed.
Stirred tank bioreactors are used in many types of aerobic biological processes, including biooxidation of refractory sulfide gold ores and bioremediation of contaminated soils. Stirred tank bioreactors provide very good contact between the bioleachant and the solid material to be treated. In addition, stirred tank processes typically have favorable oxygen conditions because the tank is sparged with air or oxygen. However, even in stirred tank bioreactors where oxygen is provided by air or oxygen sparging, the low solubility of oxygen in water (10 ppm) requires a large gas-water interface. This is generally achieved with impellers and significant expenditures of energy. The high energy costs associated with stirring and aerating the reactor make this type of bioreactor primarily applicable to bioprocesses that come to a desired end point relatively quickly, typically less than a week. For slower biological processes, a low energy cost, large scale, generally static batch process, is the best solution. However, the goal of providing the bacteria, or other microorganism, with an optimal environment is still of primary importance.
There are three primary types of static batch bioreactors used to biotreat soils contaminated with toxic organic compounds. One of these methods is landfarming. This is an above grade treatment of contaminated soil in a large open space. The soil is spread over a high-density polyurethane lined area generally covered with sand to allow for drainage. Air can be introduced by perforated pipes and by tilling the soil once or twice a week. This method has been widely implemented at sites contaminated with polynuclear aromatic (PNA's) and pentachlorophenol (PCP). One limitation of this process is that a large area is needed because the soil is spread relatively thinly to ensure adequate air flow. This method also requires tilling and may be limiting in air if the layer of soil is too thick or does not mix well.
Another technology used in the bioremediation of contaminated soil is composting. The compost is made up of contaminated soil and various amendments necessary for composting to be sustained such as wood chips, straw, or manure. These amendments increase the amount of biodegradable organics, structurally improve the compost matrix by reducing bulk weight and increasing air voids, and increase the amount of inorganic nutrients in the mixture. The composting can be carried out in a vessel with forced air flow or in open piles that are aerated by air pipes or by tilling. One disadvantage to the addition of organic amendments is that their biodegradation generates heat and requires oxygen. Composting is usually run in batch mode and a portion of the compost is used to inoculate the next compost. This process has been used effectively on many types of organic contaminates including diesel fuel, 2,4,6 trinitrotoluene (TNT), polyaromatic hydrocarbons (PAH), benzene, and xylene.
Heap bioremediation is another static bioprocess used in the bioremediation of excavated contaminated soil. In this process the soil is placed in piles 8 to 12 feet high over a lined area. To improve air flow, air can be introduced by perforated pipes. In such circumstances, the pipes are placed on approximately a 12 inch bed of the contaminated soil in regular intervals. The pipes are then typically covered with a layer of gravel to protect them from the heavy equipment. The excavated soil is then dumped in an 8 to 12 foot high pile on top of the gravel. Moisture is maintained with an irrigation system. The soil may need fertilizer or lime to adjust pH and may need sand to increase porosity. This process is low cost and thus is applicable to slow biological processes. However, this process may be too slow if the heap becomes air limited due to compaction of the soil during or after pile construction.
Therefore, air and liquid access remain important rate limiting considerations in existing static batch bioprocesses used for soil remediation, such as heap pile bioremediation, composting and landfarming. Air flow is improved in existing processes to the extent possible by introducing air through perforated air pipes or by tilling the soil. However, any flow constriction within the bioreactor will interfere with the efficiency of the process. Also, if parts of the contaminated soil are not exposed to bacteria or other nutrients as well as oxygen, the overall bioprocess will be slowed or not proceed to completion. Similarly, in the case of heap biooxidation of coal and refractory sulfide gold ore, biooxidation of the sulfides is efficiently carried out by the bacteria only when the metal sulfides are exposed to bacteria, water, nutrients, and air. If the sulfides are buried in the ore or in the solid pieces of coal, the biooxidation will not proceed. In addition, if air or liquid flow in the heap becomes limited, the biooxidation will also become limited. Consequently, a need exists for an improved bioreactor design that will permit the biotreatment of solid materials with improved air and fluid flow throughout the bioreactor and the solid material to be treated.
The use of acidophilic, autotrophic bacteria to biooxidize sulfide minerals in refractory sulfide ores is one biotreatment that has gained particular vigor in the last ten to twenty years.
Gold is one of the rarest metals on earth. Gold ores can be categorized into two types: free milling and refractory. Free milling ores are those that can be processed by simple gravity techniques or direct cyanidation. Refractory ores, on the other hand, are not amenable to conventional cyanidation treatment. Gold bearing deposits are deemed refractory if they cannot be economically processed using conventional cyanide leaching techniques because insufficient gold is solubilized. Such ores are often refractory because of their excessive content of metallic sulfides (e.g., pyrite and arsenopyrite) and/or organic carbonaceous matter.
A large number of refractory ores consist of ores with a precious metal such as gold occluded in iron sulfide particles or other metal sulfide particles. The iron sulfide particles consist principally of pyrite and arsenopyrite. Precious metal values are frequently occluded within the sulfide mineral. For example, gold often occurs as finely disseminated sub-microscopic particles within a refractory sulfide host of pyrite or arsenopyrite. If the gold, or other precious metal, remains occluded within the sulfide host, even after grinding, then the sulfides must be oxidized to liberate the encapsulated precious metal values and make them amenable to a leaching agent (or lixiviant); thus, the sulfide oxidation process reduces the refractory nature of the ore.
A number of processes for oxidizing the sulfide minerals to liberate the precious metal values are well known in the art. These methods can generally be broken down into two types: mill operations and heap operations. Mill operations are typically expensive processes having high operating and capital costs. As a result, even though the overall recovery rate is typically higher for mill type processes, mill operations are typically not applicable to low grade ores, that is ores having a gold concentration less than approximately 0.07 oz/ton. Mill operations are even less applicable to ores having a gold concentration as low as 0.02 oz/ton.
Two well known methods of oxidizing sulfides in mill type operations are pressure oxidation in an autoclave and roasting.
Oxidation of sulfides in refractory sulfide ores can also be accomplished using acidophilic, autotrophic microorganisms, such as Thiobacillus ferrooxidans, Sulfolobus, Acidianus species and facultative-thermophilic bacteria in a microbial pretreatment. These microorganisms utilize the oxidation of sulfide minerals as an energy source during metabolism. During the oxidation process, the foregoing microorganisms oxidize the iron sulfide particles to cause the solubilization of iron as ferric iron, and sulfide, as sulfate ion.
If the refractory ore being processed is a carbonaceous sulfide ore, then additional process steps may be required following microbial pretreatment to prevent preg-robbing of the aurocyanide complex or other precious metal-lixiviant complexes by the native carbonaceous matter upon treatment with a lixiviant.
As used herein, sulfide ore or refractory sulfide ore will be understood to also encompass refractory carbonaceous sulfide ores.
A known method of bioleaching carbonaceous sulfide ores is disclosed in U.S. Pat. No. 4,729,788, issued Mar. 8, 1988, which is hereby incorporated by reference. According to the disclosed process, thermophilic bacteria, such as Sulfolobus and facultative-thermophilic bacteria, are used to oxidize the sulfide constituents of the ore. The bioleached ore is then treated with a blanking agent to inhibit the preg-robbing propensity of the carbonaceous component of the ore. The precious metals are then extracted from the ore using a conventional lixiviant of cyanide or thiourea.
Another known method of bioleaching carbonaceous sulfide ores is disclosed in U.S. Pat. No. 5,127,942, issued Jul. 7, 1992, which is hereby incorporated by reference. According to this method, the ore is subjected to an oxidative bioleach to oxidize the sulfide component of the ore and liberate the precious metal values. The ore is then inoculated with a bacterial consortium in the presence of nutrients therefor to promote the growth of the bacterial consortium, the bacterial consortium being characterized by the property of deactivating the preg-robbing propensity of the carbonaceous matter in the ore. In other words, the bacterial consortium functions as a biological blanking agent. Following treatment with the microbial consortium capable of deactivating the precious-metal-adsorbing carbon, the ore is then leached with an appropriate lixiviant to cause the dissolution of the precious metal in the ore.
Oxidation of refractory sulfide ores using microorganisms, or as it is often referred to, biooxidation, can be accomplished in a mill process or a heap process. Compared to pressure oxidation and roasting, biooxidation processes are simpler to operate, require less capital, and have lower operating costs. Indeed, biooxidation is often chosen as the process for oxidizing sulfide minerals in refractory sulfide ores because it is economically favored over other means to oxidize the ore. However, because of the slower oxidation rates associated with microorganisms when compared to chemical and mechanical means to oxidize sulfide refractory ores, biooxidation is often the limiting step in the mining process.
One mill type biooxidation process involves comminution of the ore followed by treating a slurry of the ore in a stirred bioreactor where microorganisms can use the finely ground sulfides as an energy source. Such a mill process was used on a commercial scale at the Tonkin Springs mine. However, the mining industry has generally considered the Tonkin Springs biooxidation operation a failure. A second mill type biooxidation process involves separating the precious metal bearing sulfides from the ore using conventional sulfide concentrating technologies, such as floatation, and then oxidizing the sulfides in a stirred bioreactor to alleviate their refractory nature. Commercial operations of this type are in use in Africa, South America and Australia.
Biooxidation in a heap process typically entails forming a heap with crushed refractory sulfide ore particles and then inoculating the heap with a microorganism capable of biooxidizing the sulfide minerals in the ore. After biooxidation has come to a desired end point, the heap is drained and washed out by repeated flushing. The liberated precious metal values are then ready to be leached with a suitable lixiviant.
Typically precious metal containing ores are leached with cyanide because it is the most efficient leachant or lixiviant for the recovery of the precious metal values from the ore. However, if cyanide is used as the lixiviant, the heap must first be neutralized.
Because biooxidation occurs at a low, acidic pH while cyanide processing must occur at a high, basic pH, heap biooxidation followed by conventional cyanide processing is inherently a two step process. As a result, processing options utilizing heap biooxidation must separate the two steps of the process. This is conventionally done by separating the steps temporally. For example, in a heap biooxidation process of a refractory sulfide gold ore, the heap is first biooxidized and then rinsed, neutralized and treated with cyanide. To accomplish this economically and practically, most heap biooxidation operations use a permanent heap pad in one of several ore on-ore off configurations.
Of the various biooxidation processes available, heap biooxidation has the lowest operating and capital costs. This makes heap biooxidation processes particularly applicable to low grade or waste type ores, that is ores having a gold (or an equivalent precious metal value) concentration of less than about 0.07 oz/ton. Heap biooxidation, however, has very slow kinetics compared to mill biooxidation processes. Heap biooxidation typically requires many months in order to oxidize the sulfide minerals in the ore sufficiently to permit the gold or other precious metal values to be recovered in sufficient quantities by subsequent cyanide leaching for the process to be considered economical. Heap biooxidation operations, therefore, become limited by the length of time required for sufficient biooxidation to occur to permit the economical recovery of gold. The longer the time required for biooxidation the larger the permanent pad facilities and the larger the necessary capital investment. At mine sites where the amount of land suitable for heap pad construction is limited, the size of the permanent pad can become a limiting factor in the amount of ore processed at the mine and thus the profitability of the mine. In such circumstances, rate limiting conditions of the biooxidation process become even more important.
The rate limiting conditions of the heap biooxidation process include inoculant access, nutrient access, air or oxygen access, toxins build up, and carbon dioxide access, which are required to make the process more efficient and thus an attractive treatment option. Moreover, for biooxidation, the induction times concerning biooxidants, the growth cycles, the biocide activities, viability of the bacteria and the like are important considerations because the variables such as accessibility, particle size, settling, compaction and the like are economically irreversible once a heap has been constructed. This is because heaps cannot be repaired once formed, except on a limited basis.
Ores that have a high clay and/or fines content are especially problematic when processing in a heap leaching or heap biooxidation process. The reason for this is that the clay and/or fines can migrate through the heap and plug channels of air and liquid flow, resulting in puddling; channeling; nutrient-, carbon dioxide-, or oxygen-starving; uneven biooxidant distribution, and the like. As a result, large areas of the heap may be blinded off and ineffectively leached. This is a common problem in cyanide leaching and has lead to processes of particle agglomeration with cement for high pH cyanide leaching and with polymers for low pH bioleaching. Polymer agglomerate aids may also be used in high pH environments, which are customarily used for leaching the precious metals, following oxidative bioleaching of the iron sulfides in the ore.
Biooxidation of refractory sulfide ores is especially sensitive to blocked percolation channels by loose clay and fine material because the bacteria need large amounts of air or oxygen to grow and biooxidize the iron sulfide particles in the ore. Air flow is also important to dissipate heat generated by the exothermic biooxidation reaction, because excessive heat can kill the growing bacteria in a large, poorly ventilated heap.
The methods disclosed in U.S. Pat. No. 5,246,486, issued Sep. 21, 1993, and U.S. Pat. No. 5,431,717, issued on Jul. 11, 1995 to William Kohr, both of which are hereby incorporated by reference, are directed to increasing the efficiency of the heap biooxidation process by ensuring good fluid flow (both gas and liquid) throughout the heap.
Ores that are low in sulfide or pyrite, or ores that are high in acid consuming materials such as calcium carbonate or other carbonates, may also be problematic when processing in a heap biooxidation. The reason for this is that the acid generated by these low pyrite ores is insufficient to maintain a low pH and high iron concentration needed for bacteria growth.
Solution inventory and solution management also pose important rate limiting considerations for heap biooxidation processes. The solution drained from the biooxidation heap will be acidic and contain bacteria and ferric ions. Therefore, this solution can be used advantageously in the agglomeration of new ore or by recycling it back to the top of the heap. However, toxic and inhibitory materials can build up in this off solution. For example, ferric ions, which are generally a useful aid in pyrite leaching, are inhibitory to bacteria growth when their concentration exceeds about 30 g/L. Other metals that retard the biooxidation process can also build-up in this solution. Such metals that are often found in refractory sulfide ores include arsenic, antimony, cadmium, lead, mercury, and molybdenum. Other toxic metals, biooxidation byproducts, dissolved salts and bacterially produced material can also be inhibitory to the biooxidation rate. When these inhibitory materials build up in the off solution to a sufficient level, recycling of the off solution becomes detrimental to the rate at which the biooxidation process proceeds. Indeed, continued recycling of an off solution having a sufficient build-up of inhibitory materials will stop the biooxidation process altogether.
The method disclosed in U.S. patent application Ser. No. 08/329,002, filed Oct. 25, 1994, by Kohr, et al., hereby incorporated by reference, teaches a method of treating the bioleachate off solution to minimize the build-up of inhibitory materials. As a result, when the bioleachate off solution is recycled to the top of the heap, the biooxidation rate within the heap is not slowed, or it will be slowed to a lesser degree than if the off solution were recycled without treatment.
While the above methods have improved the rate at which heap biooxidation processes proceed, heap biooxidation still takes much longer than a mill biooxidation process such as a stirred bioreactor. Yet, as pointed out above, with low grade refractory sulfide ores, a stirred bioreactor is not a viable alternative due to its high initial capital cost and high operating costs. A need exists, therefore, for a heap bioleaching technique that can be used to biooxidize precious metal bearing refractory sulfide ores and which provides improved air and fluid flow within the heap. In addition, a need exists for a heap bioleaching process in which ores that are low in sulfide minerals, or ores that are high in acid consuming materials such as calcium carbonate, may be processed.
A need also exists for a biooxidation process that can be used to liberate occluded precious metals in concentrates of refractory sulfide minerals. Mill processes that are currently used for oxidizing such concentrates include bioleaching in a stirred bioreactor, pressure oxidation in an autoclave, and roasting. These mill processes oxidize the sulfide minerals in the concentrate relatively quickly, thereby liberating the entrapped precious metals. However, unless the concentrate has a high concentration of gold, it does not economically justify the capital expense or high operating costs associated with these processes. And, while a mill bioleaching process is the least expensive mill process in terms of both the initial capital costs and its operating costs, it still does not justify processing concentrates having less than about 0.5 oz. of gold per ton of concentrate, which typically requires an ore having a concentration greater than about 0.07 oz. of gold per ton. Therefore, a need also exists for a process that can be used to biooxidize concentrates of precious metal bearing refractory sulfide minerals at a rate comparable to a stirred tank bioreactor, but that has capital and operating costs more comparable to that of a heap bioleaching process.
In addition to concentrates of precious metal bearing sulfide minerals, there are many sulfide ores that contain metal sulfide minerals that can potentially be treated using a biooxidation process. For example, many copper ores contain copper sulfide minerals. Other examples include zinc ores, nickel ores, and uranium ores. Biooxidation could be used to cause the dissolution of metal values such as copper, zinc, nickel and uranium from concentrates of these ores. The dissolved metal values could then be recovered using known techniques such as solvent extraction, iron cementation, and precipitation. However, due to the sheer volume of the sulfide concentrate formed from sulfide ores, a stirred bioreactor would be prohibitively expensive, and standard heap operations would simply take too long to make it economically feasible to recover the desired metal values. A need also exists, therefore, for an economical process for biooxidizing concentrates of metal sulfide minerals produced from sulfide ores to thereby cause the dissolution of the metal values so that they may be subsequently recovered from the bioleachate solution.
Therefore, while a need exists for a method of biooxidation that can be used to process sulfide concentrates from refractory sulfide ores at a rate which is much faster than that of existing heap biooxidation processes, yet which has initial capital costs and operating costs less than that of a stirred bioreactor, this need has gone unfulfilled. Further, while a need has also existed for a method of biooxidation that can be used to economically process sulfide concentrates of metal sulfide type ores, this need has also gone unfulfilled.